1. Field of the Invention
This invention relates to a water-proof support of photographic paper and a process for the preparation of the same.
2. Description of Prior Arts
A photographic paper consists essentially of a support, a silver halide emulsion layer (i.e, photographic emulsion layer), and a protective gelatin layer. In the case that a paper sheet made of pulp is employed as the support, the paper sheet ought to be so processed as to keep itself from permeation of a developing solution. In the prior art, the paper sheet is generally water-proofed on its both sides by forming a water-proof polymer layer of a polyolefin resin such as polyethylene. The formation of the polyolefin resin layer is generally done by coating a melted polyolefin resin through melt extrusion on both surfaces of the paper sheet and chilling the coated resin by means of a cooling roll. The front polyolefin resin layer to receive thereon a photographic emulsion generally contains a white pigment such as titanium dioxide to improve hiding powder of the layer.
As is stated above, the ordinary support on photographic paper is composed essentially of a paper sheet, a white pigment-containing polyolefin resin layer provided on one side of the sheet to receive a photographic emulsion layer, and a polyolefin resin layer possibly containing a white pigment therein provided on another side of the sheet. The polyolefin resin layer of a support of this type is preferably as thin as possible from the viewpoint of production efficiency and production cost. However, if the white pigment-containing resin layer is made thinner, the hiding power of the resin layer lowers. Therefore, the thin polyolefin resin layer has to contain a white pigment therein at an increased density.
However, since a melted polyolefin resin is so viscous as to poorly disperse the white pigment therein, the increase of the density (or concentration) of a white pigment in the polyolefin resin layer is under certain limitation. At present, a white pigment-containing polyolefin resin layer of the support contains a white pigment almost at the maximum level. Accordingly, it is difficult to make thinner the white pigment-containing polyolefin resin layer without decreasing the total amount of white pigment in the resin layer. The decrease of the total amount of the white pigment results in decrease of hiding powder thereof. A photographic paper using a support having such poor hiding power hardly gives a photographic print of high resolution.
Further, a melted polyolefin resin sometimes turns yellow in the coating procedure according to melt extrusion or brings pin holes into the resulting resin layer. whereby reducing the quality of the obtained support.
In view of the above problems, Japanese Patent Provisional Publications No. 57(1982)-27257 and No. 57(1982)-49946 have proposed processes of preparing a water-proof resin layer which comprises coating a polymerizable organic compound on a support and curing thus coated layer by irradiation with electron beam. These processes use a coating dispersion of a white pigment dispersed with high concentration in a solution of low viscosity which is made by dissolving an organic compound having a unsaturated bonding in the molecular structure in a solvent. The coating solution is coated on a paper sheet and the resulting coated layer is then cured by irradiation with electron beam to form a polymer layer. This process is advantageous in that a thin water-proof resin layer can be formed without reduction of hiding powder, because a white pigment can be uniformly introduced into a coating solution with high concentration. Further, since the resin layer is formed by irradiation with electron beam at an ambient temperature, the formed resin layer is almost free from yellowing caused by thermal decomposition and further almost free from formation of pin holes.
It has been noted by the present inventors that the support of photographic paper prepared according to the above-mentioned known processes is liable to adsorb a developing agent thereon in the developing stage and to be colored to yellow by action of the adsorbed developing agent. Further noted is that cracks are easily produced in the white pigment-containing resin layer of the support, when the support is bent.